Geologic formations are used for many purposes such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. Knowing the density of a formation as a function of depth is very important because this information can be used to determine the lithology or rock type and porosity of the rocks making up the formation.
Typically, formation density is measured with a logging tool that is conveyed in a borehole penetrating the formation. The tool includes a gamma-ray source, which is normally Cs-137 emitting 0.662 MeV gamma-rays, and two collimated gamma-ray detectors at different distances from the source. The detectors are used to measure gamma-rays produced mostly by Compton scattering interactions between the source gamma-rays and the formation elements and provide data to determine the formation density. This type of logging tool requires a very high-intensity chemical source emitting 2-3 Curies of radiation. Unfortunately, there are several types of challenges associated with handling and using this type of chemical source. Hence, it would be well received in the drilling and exploration industry if density logging could be made without these types of challenges.